Ever more powerful machines, such as for example generators, are being developed, because advancing technology demands ever higher power densities. A powerful generator, such as for example a turbo generator, has in particular a stator with a laminated stator core and a plurality of generator slots in which the generator winding is located.
The main insulation of this winding with respect to the laminated core is a system that is subjected to great electrical loading.
High voltages occur during operation and have to be brought down in the insulating volume between the conductor bar, which is under a high voltage, and the laminated core, which is at ground potential. Increases in the field thereby occur at the edges of the sheets in the laminated core, which for their part cause partial discharges. When they meet the insulating system, these partial discharges lead locally to very intense overheating. In this case, the organic materials of the insulating system, including those of the outer corona shielding system, are successively broken down into low-molecular, volatile products, for example CO2.
An important component part of the insulating system is the so-called outer corona shielding (OCS). In the case of relatively large generators and electric motors, it is applied directly to the surface of the winding insulation. The OCS currently consists of corona shielding papers containing carbon black and graphite.
Because, for system-related reasons, the interface between the OCS and the main insulation especially cannot be produced completely without pores, correspondingly high electrical field intensities in the insulating system cause correspondingly high electrical partial discharge activity, which during operation completely incinerates the outer corona shielding over time and consequently leads to premature aging of the insulation and in the worst case to a ground fault of the electrical machine. This corresponds to an irreparable complete failure of the machine.
The outer corona shielding must have a certain square resistance, which lies in a certain range. If it is too low, the laminated cores may be electrically shorted, which can lead to high induced circulating currents, which are manifested over the ends of the laminated core and the outer corona shielding and lead to high-current arcs. If the resistance is too high, high-voltage spark erosion may in turn occur. It would be ideal if the resistance in the outer corona shielding system could be set, so that it would be possible to establish an anisotropy that displays increased conductivity in the radial direction, that is to say from the current-carrying conductor to the laminated core, and increased resistance, that is to say low conductivity, in the direction of the bar.